Dental plaque forms as a film on teeth. It is a product of microbial growth, a dense microbial layer consisting of a mass of microorganisms embedded in a matrix, which accumulates on the tooth surfaces. The microorganisms are mainly coccoidal organisms, particularly in early plaque, which, in the mouths of some persons at least, change to filamentous organisms after a few days.
Dental plaque has been observed to form following a dental prophylaxis treatment, due to bacteria which grew out of defects in the tooth enamel where they had resided and remained unaffected by the prophylaxis treatment. Plaque may form on any part of the tooth surface, and is found particularly at the gingival margin, in cracks in the enamel, and on the surface of dental calculus. The importance of giving consideration to the action of plaque on the teeth lies in the tendency of plaque to produce gingivitis and perhaps other types of periodontal disease, as well as dental caries and dental calculus. Dental plaque is a precursor of dental calculus. The latter forms from the plaque that accumulates on the teeth in the form of a hard mineralized deposit. It is particularly prone to form at the gingival margin, i.e., the junction of the tooth and gingiva. Both the bacterial and non-bacterial components of plaque are mineralized to form calculus, which comprises, in addition to mineralized bacteria, organic constituents, such as epithelial cells, live bacteria, salivary proteins, leucocytes, and crystals of substances having molecularly bound calcium and phosphorus, e.g., hydroxyapatite, octacalcium phosphate, brushite and whitlockite. Calculus, like plaque, is considered to be a prime causative factor in periodontal disease.
Therapeutic compositions containing metal oxides, including zinc oxide, within polymer particles have been described. For example, U.S. Pat. No. 6,368,586 discloses methods and compositions in which metal oxides, including zinc oxide, are incorporated into polymer microaggregates that contain a therapeutic ingredient in order to enhance the adhesion of the therapeutic polymer to mucosal surfaces. Spray drying and several alternative routes for producing polymer/metal oxide particles are disclosed. U.S. Pat. No. 7,691,413, describes composite particles containing a polyolefin-based resin having a crystallization degree of 80% or less and zinc oxide which are obtained by hot melt microencapsulation or spray cooling, a process for producing the composite particles, and cosmetics containing the composite particles.
Zinc oxide and zinc salts are known to be useful ingredients in oral care products. For example, U.S. Pat. No. 5,486,350 describes zinc compounds, including zinc oxide, among a list of anti-tartar or anti-calculus compounds useful in toothpaste. Zinc oxide has also been dispersed within a polymer matrix, cast into film and divided into strips for incorporation into oral care compositions. It is challenging to formulate zinc oxide in oral care products, however, as the particles tend to agglomerate and precipitate out of the formulation (zinc oxide is nearly insoluble in water) and/or react with other ingredients.